Title of the grant Signal transduction mechanisms that mediate normal and pathologic angiogenesis Abstract Pulmonary arterial hypertension (PAH) is a progressive disease, characterized by vasoconstriction, cell proliferation, and fibrosis, leading to elevated pulmonary arterial pressure and often causing right heart failure and death. There is no cure for this disease. Therefore, novel mechanistic studies and new therapeutic strategies are urgently needed. Elevation of plasma cytokines in PAH patients is a hallmark of inflammation. As the major effector, monocytes release cytokines and infiltrate in perivascular regions of the lung. Depletion of monocytes attenuates vascular remodeling and hemodynamic changes in PAH animal models. This evidence implies a communication between monocytes and endothelial cells (ECs). However, the underlying mechanisms are not well understood. The objective of the current proposal is to define the role of extracellular domain of cleaved delta like 4 (exDll4) in the pathogenesis of PAH and its mechanisms. In vitro, we discovered that calpain1 can cleave Dll4. We also found that TNF? and IL-1? increases Dll4 expression, Dll4 cleavage in human monocyte, and exDll4 release from monocytes. Furthermore, we found that recombinant exDll4 significantly increased apoptosis and decreased barrier function in ECs. Seeking the mechanism of action, we found exDll4 associated with intact Dll4, and this interaction prevents Dll4 binding to Notch1 and thus inhibits Notch1 activation. In vivo, we found that Dll4 expression and exDll4 release in monocyte are significantly elevated in PAH mice as well as in PAH patients. However, Notch1 signaling is decreased in lungs during PAH progression in mice and rats. Based on these findings, we hypothesize that exDll4 derived from monocytes is crucial for PAH progression by inducing apoptosis and impairing barrier function in lung EC. Mechanistically, exDll4 forms a heterodimer with Dll4 to prevent the association of Dll4 and Notch1, blocking Notch1 signaling. To test our hypothesis, we propose three aims. Aim 1. Define the regulatory mechanisms of Dll4 expression and exDll4 release from monocyte in PAH. Aim 2. Determine the biological function of exDll4-Notch1 in lung EC and its molecular mechanisms. Aim 3. Characterize the therapeutic effects of inhibiting monocyte Dll4 on PAH progression. Accomplishing these aims will 1) fill the knowledge gap regarding the mechanisms of vascular remodeling and EC dysfunction in the pathogenesis of PAH mediated by monocyte derived exDll4; 2) reveal the regulation of Dll4 cleavage; and 3) invent a novel therapeutic strategy for PAH by targeting monocyte Dll4.